1. Field of the Invention
The present invention is related to a method of producing a diene polymer by which the polymer comprising a high molecular weight polymer containing the chain having a tin-carbon linkage as the main component thereof and consequently having low hysteresis loss can be obtained with excellent productivity and inexpensively. The present invention is also related to a rubber composition for tires having excellent wear resistance, low rolling resistance and excellent wet skid resistance without adversely effecting on fracture properties by using the diene polymers.
2. Description of Related Art
Because the reduction of fuel cost of automobiles is required, rubbers having low hysteresis loss are desired for material rubbers of tread parts of tires. As the rubber materials having low hysteresis loss, natural rubber, cis-1,4 polyisoprene and low cis-1,4 or high cis-1,4 polybutadiene are known. As rubber materials in which the low hysteresis loss property is remarkably improved, polymers obtained by coupling a halogenated tin compound to the end of the polymer prepared by polymerization with an organolithium compound as the initiator in a hydrocarbon solvent are known (Japanese Patent Application Laid Open No. 1982-55912 and others). In the method of producing these polymers, when a halogenated tin compound is brought into reaction with the end of the polymer, the polymerization system is converted to the deactivated state at the time of the addition of the tin compound and the residual monomer is considered to be left remaining without reaction. Thus the molecular structure of the polymer obtained is fixed to a certain structure at the time of the addition of the tin compound, and the polymer has the molecular structure containing tin in the chain. Therefore, the tin compound is generally added at the time when the polymerization is finished because of the requirement of the reaction which is related to the economy of the production and the requirement of the molecular structure which is related to the molecular design. As described above, in the heretofore known methods of production by using a modifier such as coupling agents like halogenated tin compounds, the modifier is added after the reaction is finished, and, even when the unreacted monomer is remaining, the reaction system is sent to the following process to recover the polymer of the purpose. This means that a batch process is adopted as the reaction process. However, the batch process has lower productivity than the continuous process, and the production cost of the polymer is increased to cause a higher price of the polymer, leading to a major drawback.
On the other hand, when the continuous process of polymerization, which is well known as an inexpensive method, is assumed to be the primary factor to be considered, some such methods can be found. As the first of such methods, the method of producing a blend of diene rubbers by using an organolithium compound as the initiator is mentioned (Japanese Patent Application Laid Open No. 1988-235305). In this method, a branching agent such as tin tetrachloride is added in the middle period of the copolymerization of butadiene and styrene (30 to 70% conversion) to form a low molecular weight polymer containing the chain having a tin-carbon linkage by the coupling of 20 to 70% of the polymer and a high molecular weight polymer containing no chain having tin-carbon linkage which is formed by the continued polymerization with the remaining active ends. Further, a polymer blend containing the high molecular weight polymer as the main component is formed. This method of production has the object of obtaining an inexpensive polymer having the improved processability by producing a low molecular weight polymer and a high molecular weight polymer in the process of the polymerization instead of using the ordinary method of blending.
As the second of such methods, the method of producing polybutadiene or polyisoprene for improvement of processability is mentioned (U.S. Pat. No. 3,536,691). This method has the object of obtaining the polymer having improved processability by adding allyltin as the modifier in the middle period of the polymerization when a monomer is polymerized in the presence of a special kind of lithium compound which is a haloaryllithium. Like the method described above, this method does not positively introduce the chain having a tin-carbon linkage into the molecular chain of a high molecular weight polymer, and the object is not to obtain a polymer having low hysteresis loss.
As still another method, the method of producing conjugated diene polymers for improvement of cold flow of the polymer can be mentioned (Japanese Patent Publication 1966-9433). In this method, a compound having the formula R'.sub.4 M (wherein R' is a vinyl group, an alkyl group, a cycloalkyl group or an aryl group, at least two of the four groups are vinyl groups and M is silicon, germanium, tin or lead) is added to the polymerization mixture before completing the polymerization in the method of producing polybutadiene or polyisoprene in the presence of an organometallic compound of lithium or aluminum or a catalyst containing metallic lithium. As R'.sub.4 M, for example, divinyldimethylsilane and the like are used, and these compounds are generally known to react as divinyl compounds like divinylbenzene to cause the crosslinking reaction when they are used in the anionic polymerization. This method is used for improving the cold flow by introducing a branched structure in the main chain of polymer by using R'.sub.4 M and not for positively introducing the chain having a metal-carbon linkage.
These examples show that, though methods of the continuous polymerization are known, a method of producing an excellent polymer having low hysteresis loss by the continuous process has not been known.
Because low fuel cost and safety are both required for automobiles, improvement of low rolling resistance (low hysteresis loss), wear resistance and wet skid resistance are desired for rubber compositions used for the tread part of tires.
For decreasing the rolling resistance of rubber compositions for the tread part, properties of the polymer comprised in the rubber composition were focused upon, and natural rubber, polyisoprene rubber, polybutadiene rubber and the like have heretofore been used as the low hysteresis loss polymer. However, the following problem arose because rolling resistance and wet skid resistance are contradicting properties: when a rubber composition containing these polymers were used, the wet skid resistance of a tire was decreased in the condition where the rolling resistance could be decreased, and on the other hand, the rolling resistance of the rubber composition could not be decreased when the wet skid property was improved because the amounts of compounding of reinforcing agents like carbon black and plasticizers like oil had to have been increased for the improvement.
For creating a proper balance between wear resistance, rolling resistance and wet skid property, blending of polymers having different content of the bound styrene or the content of the vinyl linkage in the butadiene part was examined as the styrene-butadiene copolymer comprised in the rubber composition (Japanese Patent Applications Publication Nos. 1971-28069, 1972-42729, 1972-13532, 1972-17449 Japanese Patent Application Laid-Open Nos., 1980-60539, 1981-163908, 1982-70137 and 1982-55941). However, the increase of the content of the bound styrene increased the wet skid resistance but did not decrease the rolling resistance.
Furthermore, when the vinyl content of the butadiene part of styrene-butadiene copolymer in the rubber composition was increased, the fracture properties and the wear resistance were decreased even though the wet skid property of the tire using this rubber composition improved. Therefore, when the above descriptions are considered together, optimization of the content of the bound styrene, the microstructure of the butadiene part, the distribution of components in the molecular chain and the molecular weight distribution alone are insufficient for obtaining a rubber composition favorable for a tire tread satisfying the requirement of low fuel cost which corresponds to the requirement of low rolling resistance (low hysteresis loss) of the rubber compounds.